Massive neutrino decay and the photoionization of the intergalactic medium

It is pointed out that the intergalactic medium might be ionized by photons emitted by a cosmological distribution of massive neutrinos. The absence of absorption troughs in quasar spectra due to atomic hydrogen and helium, and the possible presence of a trough due to singly ionized helium, would then imply that the neutrino mass lies between 50 and 110 eV. Its radiative lifetime would have to be 10²⁷ s if the critical ionization redshift ∼ 3.5 and the present density of the IGM $$\sim5\times10^{-9} \text{cm}^{-3}$$. The apparent quasar cut-off at a redshift ∼ 3.5 may be due to the IGM being neutral at higher redshifts. The inferred lifetime could be compatible with particle physics expectations if the GIM mechanism is not operating.